Surfactants are commonly used by those skilled in the art in emulsion polymerization processes to stabilize organic species such as monomer molecules or polymer particles in an aqueous medium. These surfactants have a quite specific structure, since they have on the same backbone a hydrophilic species and a hydrophobic species. In general, small ionic or nonionic molecules are the surfactants most commonly used in emulsion polymerization. Mention will be made, in a non-exhaustive manner, of sodium lauryl sulphate (SLS) and sodium dodecylbenzenesulphonate (NaDDBS).
However, the use of such surfactants may generate problems in the final application of latices. The reason for this is that these surfactant molecules generally of low molar mass have a tendency to migrate to the polymer-substrate interface, which is generally reflected by an impairment in the properties of the films or particles produced with the latices.
Thus, mastering the properties of the surfactant makes it possible to control the application properties of latices, for instance control of the viscosity of the latex, or control of the “plate-out” phenomena (formation of deposits on extrusion tools), which is of interest in various fields of use of latices, such as paint formulations, plastics additives or cosmetic formulations.
In order to circumvent the difficulties mentioned above, alternative methods, for instance the use of reactive surfactant molecules, have been used. This method considerably increases the performance qualities of the latex due to the capacity of the surfactant to react covalently with the monomer. With the surfactant thus attached to the surface of the polymer particles, all problems of migration are thus avoided.
Still in the perspective of minimizing the migration of the surfactant, an alternative method consists in using macromolecular surfactants in the emulsion polymerization process. Due to their macromolecular nature, these polymeric surfactants make it possible to overcome the problems mentioned above associated, in the majority of cases, with the migration of small molecules.
These macromolecular surfactants are amphiphilic copolymers that combine a hydrophilic species and a hydrophobic species chemically bonded together on the macromolecular backbone.
The amphiphilic copolymers commonly used as surfactants in emulsion polymerization are block, random, grafted or alternating copolymers or alternatively star copolymers.
These macromolecular stabilizers may be synthesized via various polymerization techniques such as anionic polymerization, standard free-radical polymerization or controlled free-radical polymerization.
The amphiphilic copolymers derived from standard free-radical polymerization are random copolymers more generally grouped under the term ASR (meaning alkali-soluble resin). They are formed from hydrophobic monomer(s), for instance styrene or α-methylstyrene, and from hydrophilic monomer(s), for instance acrylic acid or methacrylic acid. Examples of ASRs that may be mentioned include the Joncryl copolymers from Jonhson Polymer (styrene-acrylic resins), the Neocryl copolymers (styrene-acrylic copolymers) and Haloflex copolymers (vinyl-acrylic copolymers) from NeoResins or the Morez 101 copolymers (styrene-acrylic resins) and Tamol® copolymers from Rohm & Haas. The latter copolymers may be copolymers of diisobutylene and of maleic acid or alternatively copolymers of maleic anhydride sodium salts.
Other examples of commonly used amphiphilic copolymers are the SMA® products produced and sold by SARTOMER. These are styrene-maleic anhydride copolymers with a molar ratio of these two monomers of between 1:1 and 4:1.
The examples of emulsion polymerization describing the use of such amphiphilic copolymers as surfactants show that these copolymers are generally not used alone, but in combination with surfactant molecules of low molar mass (U.S. Pat. No. 4,529,787, U.S. Pat. No. 4,414,370, U.S. Pat. No. 6,160,059).
When the amphiphilic copolymers mentioned above are used as sole emulsion polymerization surfactants, a major drawback lies in the need to introduce large amounts of them in order to obtain stable latices (up to 50% by weight relative to the weight content of monomers). The reason for this is that, due to the composition polydispersity of the macromolecular chains directly associated with the free-radical polymerization process that is well known to those skilled in the art, an appreciable number of polymer chains do not participate efficiently in stabilization of the latex. Even though, overall, the polymer derived from the process comprises a hydrophobic/hydrophilic ratio in proportions adequate for the surfactant property desired according to the application, the distribution of these units is not uniform in the polymer chains. These chains are then either too hydrophilic (dissolution in the aqueous phase) or too hydrophobic (dissolution in the monomers) to be present at the aqueous phase/organic phase interface as required for the application. To understand the role of the polymerization process on the distribution of the monomers in the polymer chains, reference may be made to the publication by B. Charleux (Macromol. Symp. 2002, 182, 249-260), which deals with the case of hydrophobic monomers, but which may also be generalized to the case of hydrophilic/hydrophobic monomer mixtures.
One method for overcoming the problems of homogeneity of composition of polymer chains that is well known to those skilled in the art is the Controlled Free-Radical Polymerization process (generally referred to as CFRP). Thus, the copolymerization of a hydrophilic monomer and a hydrophobic monomer according to the CFRP process leads to an amphiphilic copolymer in which the chemical composition of the polymer chains is uniform and similar from one chain to another. Under these conditions, a majority of macromolecular chains participate in stabilizing the latex since the composition is suited to the surfactant property of the copolymer.
In the field of amphiphilic copolymers derived from Controlled Free-Radical Polymerization, the prior art reports the use of structured materials, generally block copolymers (FR 2 838 653, WO 2002/068550, WO 2002/068487, DE 196 54 168, DE 197 04 714, DE 196 02 538, Polymeric Materials Science and Engineering 1998, 79 440-441).
These materials have the advantage of forming micellar aggregates in certain solvents. These micelles may then serve as sites for creating particles. The efficacy of block copolymers as surfactants has moreover already been demonstrated. Examples that will be mentioned include the case of emulsion polymerization of a methyl methacrylate/butyl acrylate mixture of 35/65 mass ratio containing 45% solids, in which the use of only 0.15% by weight of polystyrene-b-sodium polyacrylate block copolymer (in which the polystyrene block has a degree of polymerization of 10 and the poly(sodium acrylate) block has a degree of polymerization of 56), relative to the weight content of monomers makes it possible to obtain a stable latex with a mean particle diameter of about 156 nm.
However, these copolymers suffer from a preparation process that is often long and expensive, which involves a multi-step synthesis. Specifically, the preparation of block copolymers involves a sequence of at least two polymerization steps (successive construction of the polymer blocks) between which is a step or devolatilization of the residual monomers present at the end of the first step. Furthermore, until very recently, the controlled free-radical polymerization techniques proposed to those skilled in the art, such as ATRP (meaning Atom-Transfer Radical Polymerization) and NMP (meaning Nitroxide-Mediated Polymerization) did not allow the direct polymerization of functionalized monomers such as acrylic acid or methacrylic acid. The introduction of the acrylic unit into the chain thus required an additional step of acidolysis of copolymers based on tert-butyl acrylate.